<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <button>ajax</button>
  <script>
    const btn = document.querySelector('button')
    btn.onclick = function() {
      fetch('http://127.0.0.1:8000/server?a=123&b=123', {
        method: 'post',
        headers: {
          'Content-Type': 'application/x-www-form-urlencoded'
        },
        body: 'username=123&password=123'
      }).then(
        response => {
          return response.text()
        }
      ).then(
        response => console.log(response)
      )
    }
  </script>
</body>
</html>